Materials analysis is an important technology for many industries, including the integrated circuit fabrication industry, where the ability to confirm the stoichiometry of the various layers that are formed during the fabrication of an integrated circuit is essential. Analysis techniques generally distinguish elements based upon unique properties of the elements, such as molecular weight. For a variety of reasons, relatively lighter molecular weight elements, otherwise referred to herein as light elements, tend to be somewhat more difficult to detect. As used herein, light elements have a molecular weight of no more than about twenty-one atomic mass units.
One method of measuring relatively light molecular weight elements in thin films is to use a wavelength dispersive x-ray detector tuned to the characteristic x-ray line of the desired light element. Another method is to use an energy dispersive x-ray detector to detect the entire spectrum of x-rays including medium and heavy elements, as well as the continuous bremsstrahlung radiation.
The wavelength dispersive x-ray method suffers from a limited solid angle that can be collected due to the geometry necessary to satisfy the Bragg condition for the reflection of x-rays. Another disadvantage of the wavelength dispersive x-ray method is that the Bragg reflector generally has a low efficiency, usually less than about ten percent. A third disadvantage of the wavelength dispersive x-ray method is the inability to measure the continuous x-ray background at neighboring wavelengths. It is necessary to add a second wavelength dispersive x-ray detector to measure the background at a single neighboring wavelength, which does not always yield adequate information.
Energy dispersive x-ray analysis characterizes materials by exciting a sample with ionizing radiation. An energy-dispersive x-ray analyzer is a common accessory for a scanning electron microscope. The electron beam in the scanning electron microscope typically has an energy of between about five thousand and about twenty thousand electron volts, and provides the ionizing radiation. The binding energy in an atom ranges from a few electron volts up to many thousand electron volts. Atomic electrons are dislodged as the incident electrons from the scanning electron microscope beam pass through the sample, thus ionizing atoms of the sample.
After an atomic electron is ejected from the sample, another electron, such as from a nearby atom, neutralizes the ionized atom. This neutralization produces an x-ray with an energy level that is characteristic of the sample atom. Another mechanism, known as bremsstrahlung, also produces x-rays. In this case an electron from the beam is significantly deflected by the strong electric field of an atom's nucleus. As the electron curves around the nucleus, it emits an x-ray. These x-rays can be emitted over a wide, continuous energy range and are not characteristic of the atom which produced them. By using x-ray detection equipment to count the number of x-ray photons emitted at a given energy level, the energy dispersive x-ray system is able to characterize and quantify the elemental composition of the sample.
The energy dispersive x-ray method overcomes the problems of wavelength dispersive x-ray, but suffers from its sensitivity to all the x-rays, including bremsstrahlung from the light element atoms and both bremsstrahlung and characteristic x-rays from the medium and heavy elements. Generally the x-rays from medium and heavy x-rays are much more intense than those from the light elements, and can overwhelm the energy dispersive x-ray detector with too high a count rate.
What is needed, therefore, is a system for light element material analysis that at least reduces some of the problems with the currently used techniques.